1. Field of the Invention
The present invention relates to a system and method for measuring, a pressure and flow of blood, more particularly it is related to the perivascular measurement of blood flow and pressure at the same location on a blood vessel.
2. Background of the Invention
Blood flow and blood pressure measurement provide useful physiological information in biological systems. If flow and pressure are measured at the same location of a blood vessel, the measurement can allow a determination of the impedance of the tissue or organs to which the vessel is supplying blood.
At present localized pressure measurement in a blood vessel is commonly made with a sensor placed at the end of a catheter tip which is inserted into the blood stream. Because of the invasive nature of the catheter, and the possible change in flow and pressure that can result from introducing a foreign object into the blood stream, use of a catheter has limitations. Also chronic or long term measurements can not be made with a catheter since prolonged insertion of the catheter into the blood vessel causes the patient's immune system to treat the catheter as a foreign body and tissue will form around the catheter thus degrading the ability of the catheter to measure flow and pressure.
Another pressure measurement principle is the tonometric approach, where a pressure sensor is pressed against the outside of a vessel. If certain conditions are met, the pressure sensed in this manner will be equal to the blood pressure inside the vessel. Although the tonometric principle of blood pressure measurement is known and has found use for the non-invasive measurement of intra-arterial pressure (see for instance U.S. Pat. No. 5,284,150) tonometrics has not been adopted as an implantable method for measuring the localized blood pressure of a vessel due to a number of technical problems. A discussion of the general theory behind the technique appears in the article “Arterial Tonometry: Review and Analysis” by Drzewiecki, Melbin and Noordergraaf in the J. Biomechanics Vol. 16 No. 2 pp, 141-152 (1983).
Perivascular measurement of blood volume flow with ultrasound has been a standard technique which has been used since the 1980's. U.S. Pat. No. 4,227,407, describes a perivascular system and method of ultrasound measurement. The principles described in this patent have been applied in the development of transit time flow sensors by Transonic Systems Inc. of Ithaca, N.Y. Doppler flow velocity measurements have been documented since the 1970's, and may be used as an alternate flow measurement approach.
Thus, what is needed is a system and method to obtain in real time pressure and flow readings in a blood vessel or other type of flexible conduit. There is also a need for a system and method to obtain continuous readings of flow and pressure in a blood vessel or other type of flexible conduit over an extended period of time without loss of accuracy in the readings.